Phase modulation



June 23, 1936. H H RE 2,045,107

PHASE MODULATION Original Filed De c. 2, 1952 4 sheets-sheet 1 e m/va yam/m I l 1 l l l i v MODl/MT/NG i JOURCE J INVENTOR- HEN Y SHORE BY K I a 7 f0 501/fi6f0f056. ATTORNEY- June 23, 1936. H, SHORE I 2,045,107

PHASE MODULATION Original Filed Dec. 2, 1932 4 Sheets-Sheet 2 7'0 MODUMT/MG JWUBCE INVENTOR- f0 500m 0/ HENRY 5HORE 4100014770 By ATTORNEY- June23,1936. HSHORE 2,045,107

I PHASE MODULATION Original Filed Dec. 2, 1932 4 Sheets-Sheet 4 70 mm. MUN/P- Sax/Ref 0F 500M! MO0UZA7/A/6 dJCIZLAT/OMS Pom/mu [INVENTOR- HENRY SHORE 'A'rroRNEY- Patented June 23, 1936 UNITED STATES PHASE MODULATION Henry Shore, Elizabeth, Corporation of Americ ware N. J., assignor to Radio a, a corporation of Dela- Application December 2, 1932, Serial No. 645,436 Renewed May 3, 1934 Claims.

This invention relates to signalling and in particular to means for varying in phase the frequency of carrier oscillations at signal frequency. The phase modulating means may be applied to radio transmitters in which the modulated and amplified oscillations are radiated into a natural medium or in transmission systems wherein the modulated oscillations may be impressed for transmission on conductors or other artificial channels.

Phase modulator systems have been known heretofore. In general the phase modulating schemes known heretofore have required that either completely new installations be made including the modulating schemes, or considerable modifications be made to the existing circuits in the transmitters.

An object of the present invention is to provide a new and improved phase modulator scheme which may be applied to completely new installations or may, without modification of existing transmitters, be inserted bodily into transmission circuits now in use to adapt the same to phase modulation signalling. All that is required in modifying a transmitter now in use, as, for example, an amplitude modulated transmitter, is to remove the amplitude modulating unit. and substitute in place thereof the phase modulator circuit of the present invention.

Moreover, the phase modulator of the present invention may be used simultaneously with amplitude modulation in new installations and in amplitude modulators now in use. This is an extremely important feature since it permits multiplexing of signals on the same carrier frequency. Furthermore, the resulting apparatus is simple in nature and eflicient in operation.

The phase modulator circuit of the present invention may be introduced or inserted in the excitation leads in any of the stages of the transmitter. For example, it may be inserted between the high frequency oscillator, the modulating potential source, and the amplifier. The only essential is that'the phase modulating circuit of the present invention be followed by an amplifier of the class A type. The reason why this requirement must be met will appear more in detail hereinafter.

In its broadest aspect my phase modulating scheme comprises a bridge circuit having four arms, each arm being composed of impedances, the impedances of an opposed pair of arms b ing variable, and means for varying at the signal frequency the impedances of said arms. Now,

if a source of high frequency oscillations is connected with one pair of diagonal points of said bridge, the potentials appearing across the other pair of diagonal points of said bridge, when the impedances of said variable arms are varied at 5 signal frequency, will be phase modulated in accordance with the said signal.

One apparent advantage to be obtained by the use of my invention is that it permits rapid in- I terchange or simultaneous use of difierent types 0 of modulation and so results in a considerable saving of time and maintenance work. For example, an amplitude or frequency modulated transmitter may be readily, and with little effort, changed into a phase modulation transmitter by merely inserting therein my phase modulating bridge circuit. Subsequently the transmitter may be reconverted into an amplitude or frequency modulated transmitter by removing the bridge circuit of the present invention and restoring the original connections. Of course, both types of modulation may be used simultaneously.

Another advantage to be gained by the use of the bridge circuit phase modulating scheme of the present invention is that, since it may be substituted or inserted in transmitters now in existence, the use of phase modulated signals with all the advantages to be gained thereby does not necessitate a complete transmitting apparatus since all that is necessary is the simplified phase modulating bridge circuit of the present invention. This results in economy since a limited amount of apparatus is necessary to signal with I several types of modulation.

The novel features of my invention have been pointed out with particularity in the claims ape pended hereto.

The nature of the circuit of the present invention and the mode of operation thereof will be better understood from the following detailed description thereof and therefrom when read'in connection with the dawings, throughout which like reference characters indicate like parts, and in which:

Figure l is a bridge circuit used to illustrate the principle of the present invention; I

Figures 2, 3, 4 and 5 are circuit arrangements similar to Figure 1, which may be used in the practical application of the invention;

Figures 6 and '7 show the application of the present phase modulating scheme to a transmitter which may be an installation now in use or a new installation; While,

Figures 8 and 9 are graphs which, with Figure 1, assist in clearly disclosing the principles of my invention.

Referring to the drawings, and in particular to Figure 1 thereof, the phase modulator circuit 1 comprises a bridge circuit having four connected arms. The impedances of the arms are substantially equal numerically in their unenergized state. Two of the opposed arms may, as shown, comprise substantially equal resistances R and R, while the opposite arms may be composed of substantially equal capacities C and C. If oscillations are impressed by way of leads 3 and 5 On to opposed points 6 and 8 of the bridge circuit, potentials of amplitude equal to the amplitude of the impressed oscillations will appear across the opposed points Ill and 12 of this bridge circuit. Due to the action of resistances R and R and the capacities C and C, these oscillation potentials appearing on the opposed points ID and I2 will be shifted in phase with respect to the oscillating potentials applied on the opposed junction points 6 and 8. The circuit is not a balanced or null bridge in which there is no output when the bridge is balanced, but is a phase angle bridge in which, as indicated, the output voltage e is equal in magnitude to the input voltage E, and in which, as indicated, in Figure 1, e=E sin (wt+0). The relationship of phase shift between the input and output oscillations may be determined by the equation:

pacities C. The potential drop in R is represented in the graph by iR, which lags the voltage E. The potential drop through C has been represented by iXc and leads the voltage E. The resultant e represents the voltage appearing across the opposed points l0, I2. The resultant voltage e r is shifted with respect to the voltage E by an angle 0. Now if the angle 0 is bisected we have the following relations:

5 iR+2 R from which 3. 0 2 tan 1% Now i1=iz because Xo=Xe and R=R. (Xo=the reactance of the condensers).

Also 4. 6=i1R'('-'ji Xg)=i R+ji Xg and RjX 2 Substituting Equation No. 5 in Equation No. 4 we get +J' c) 6. e JXC [R X jZRX E R +X g R +X g From the real part of this equation we get the cosine of the phase angle, or,

In other words, E:e in absolute values but the respective voltages are of diiferent phase. This is because the bridge is not a null bridge but is a phase angle bridge, the voltage output of which is equal in magnitude to the voltage input. The voltages are out of phase except when the bridge is in a balanced state. If the impedances in either pair of opposed arms, that is, the resistances R, R or the capacities C, C, are varied in value in phase at signal frequency, it is obvious that the phase of the oscillations appearing at the output points, that is, the opposed points It] and I2, will be shifted in variable degrees at signal frequency with respect to the phase of the oscillations impressed by leads 3 and 5 on to the other points 6 and 8 of the bridge circuit. That a change in the values of the resistances R or in the capacities C of the bridge arms results in a phase shift between the input voltage E and the output voltages e has been proven graphically in Figure 9, in which the values of the applied voltage E, the potential drop iR through the resistance R, and the potential drop iXc through the capacities are the vectors of a vector system.

First, assume that the resistance R is large as compared to the capacity C. In this case the potential drop through R is represented by the vector iiRl lagging the vector E. The potential drop through C is represented by ilXC. The resultant e1 lags the vector E by the angle 01.

Now assume that the value of the resistance R has been decreased substantially (say, by modulating means) and is small as compared to C. The potential drop through resistance R still lags E but is smaller and is represented by the vector izRz. The potential drop through C is larger and again leads the voltage E. The resultant (22 now lags the vector E by an angle 02 which is considerably larger than the angle 01. This increase in phase shift between E and e is due to the modulation of the value of R and the phase shift A0 resulting from said modulation:0z-01.

In a similar manner it can be shown that modulation of the value of C will result in a phase shift between E and e. The phase shift may not be as large when C is modulated as when R is modulated.

The phase shifted oscillations may be impressed from the leads H, 13 connected with points 13, I2 to any utilization circuit.

In practice either the resistances R, R or the capacities C, C may be varied by the modulating potentials. The practical applications of my phase modulating bridge circuit will now be described.

Referring to Figure 2 of the drawings, the bridge circuit I comprises four arms, as in Figure 1. Furthermore, an opposed pair of arms includes, as shown, capacities C and C. Here, however, the resistances R, R in the other pair of arms are replaced by the impedances between the anode l2 and cathode l3 and anode l2 and cathode l3 of thermionic tubes I4 and [4' re spectively. Space current is supplied to the tube M by way of source !6 connected, as shown, between the cathode l3 and anode !2 by way of choking inductance H. In a similar manner space charge current for the tube I4 is supplied from source l6 by way of choking inductance ll. The control grid cathode circuits of tubes l4 and I l include biasing batteries [8 and. I8 which maintain the control grids) and I9 at negative potential with respect to the cathodes and also inductances and 26'. In order to vary the anode to cathode impedances of each of tubes M and M in phase atsignal frequency, modulating potentials areapplied by way of leads 2i and 22 to an inductance 23 symmetrically coupled with inductances 2s and 20'. The oscillations at signal frequency flowing in the inductance 2'3 are impressed in like phase on to inductances 2i! and 2t and from said inductances in phase on to the control electrodes l9 and l9 of tubes l4 and I4. These in-phase potentials are superimposed upon the direct current potentials applied to the control grids l9 and I9 respectively from sources l3 and I8 respectively and vary the conductivity of the tubes in phase. Now if, as in Figure 1, the carrier frequency oscillations to be modulated are applied by leads 3 and 5 to the junction points 6 and 8, potentials equal in amplitude to the applied potentials will appear at the junction points It) and I2. The potentials at it? and I2 will be shifted in phase with respect to the applied potentials. The amount of phase shift between these potentials will be determined by the amplitude of the modulating potentials acting on the conductivity of the tubes M and l2, and the rate at which the phase shifts between the applied oscillations and the oscillations appearing at I0 and I2 take place will be determined by the frequency of the modulating potentials. The phase modulated oscillations may be utilized by connecting any utilization circuit to the leads II and I2. In order to insure efficient operation of the modulating circuit, and to insure that the phase modulations on the carrier be truly representative of the signal, it is desirable that the load, that is, the current flowing in the several arms of the bridge circuit, is maintained at minimum. This requirement is satisfied in accordance with the present invention by connecting the output leads i i, is of the bridge circuit to an amplifier of what is known as the class A type. In this type of amplifier the thermionic tube is biased to such a point that litle or no grid current flows in the input circuit and, therefore, little or no load is applied on the bridge circuit which is to be connected with the gridelectrode or with the input circuit of the amplifier.

Since change in the potential applied to the control grid electrode of a thermionic tube results in change in capacity of the tube, and since change in the value of the capacities C, C in the arms of the present bridge circuit results in phase modulation of the oscillations applied, as indicated hereinbefore, the arms C, C of the bridge circuit may be replaced by the impedances of the thermionic tubes, as shown in Figure 3. In this figure, 3, the other two arms of the bridge circuit include the two resistances R and R. The bridge circuit is otherwise the same as the bridge circuit of Figure 2. Furthermore, the oscillations to be modulated and the modulating oscillations are applied in a manner similar to which they are applied to the bridge circuit of Figure 2. Moreover, the phase modulated oscillations are derived from the points it and 12, as in Figure 2, and may be applied to any work circuit by way of aclass A amplifier.

The arrangement of Figure 3 and the opera-. tion thereof will be obvious to the reader of the prior specification, and a description thereof and be obvious to one who has read the specification, a description of the a statement of the operation thereof is thought unnecessary at this point. It will be noted, however, that, since changes in capacity of the tubes due to variations in the applied modulatingpotentials' are slight, the phase shift resulting at ,5 the junction points I 0 and I 2 will be less than in the arrangement of Figure 2 Where apparatus is limited the arrangement of Figure 3 may not be as desirable as the arrangement of Figure 2; However, if the class A amplifier,.'con- 1 nected with the leads H and I3, can be followed by a frequency multiplier, the bridge circuitof Figure 3 may be more desirable than the bridge circuit of Figure 2 since frequency multiplication of the modulated carrier will increase the i5 phase modulation suificiently to insure satisfactory detection of the modulated wave at thereceiving point.

Although in illustrating the present invention thermionic tubes of the triode type have been 20. used, it will be understood that tubes of any other type may be used, and in particular tubes of the screen grid type in which the screening electrode is maintained at a positive potential to eliminate coupling between the input andout- 253 put electrodes. The use of such" tubes in the bridge circuit of the present invention has been illustrated in Figure 2 wherein the screeninggrid electrodes and 3B are shown in dotted lines connected with a point on the potentialsour'ces 3. I6 and I6",

Moreover, the impedance of the tubes 14 and It may be varied by applying the modulating potentials to other electrodes therein; Forexample, the modulating potentials may be applied 3' to the screen grid electrodes 30 and 3f, as: indicated in Figure 4, or plate modulating may be utilized, as shown in Figure 5. Since the operation of the arrangements of Figures 4 and 5will thereof will not be given at this point.

Where the modulating potentials are applied to the screen grid electrodes, as in Figure 4, or to the plate electrodes, leads include radio frequency choke coils RFC and RFC, asshown.

In practice the bridge circuits of Figures 2, 3, 4 and 5 may in new installations be connected with a source of modulating potentials M and a source 501 of oscillations O and with a class A amplifying means A and radiating means RA, as shown in The phase modulator here and the rest of the apparatus may comprise a new installation. If phase modulation only is used the class A amplifier may be followed by an amplitude limiter AL, shown in dotted lines between the class A amplifier and the utilization circuit. The

latter is also used may be followed by frequency 0 amplifier A, and a utilization circuit FM and RA,

as shownin Figure 6, the modulatortherein, en 75.

foregoing 40;

as in Figure 5, the anode the frequency multiplier may be deis to be utilized with in- 701' closed in the dotted line rectangle and interposed between M and O and A, may be removed and replaced by a phase modulator bridge circuit, as disclosed in Figures 2, 3, 4 or 5. In this case, if the transmitter does not already include an amplifier of the class A type, such an amplifier may be interposed between the modulator l and the utilization circuits FM and RA, as shown.

When multiplex signalling is desired by new installation the arrangement as shown in Figure '7 may be used. Where multiplex operation is desired by transmitters now in use, as, for example, amplitude modulated transmitters, said transmitters may be modified as indicated in Figure 7. In Figure 7 the source of modulating potentials M is connected with a phase modulator I which is also energized by carrier frequencies from source 0. The phase modulated oscillations are fed from I to a class A amplifier A and from thence to an amplitude modulator AM, from which the phase modulated and amplitude modulated signals may be fed to frequency multipliers and amplifiers and radiating means and to one or more of such devices. If an installation, as for example, an amplitude modulated transmitter, is to be modified for multiplexing by adding a phase modulator thereto, the leads L, shown in dotted lines between the amplitude modulator AM and the source of oscillations 0, may be disconnected. The leads between the source of modulating potentials M and the amplitude modulator AM may be opened and phase modulator I may be connected between the modulation source M and the amplitude modulator AM. The source of oscillations 0 may now be connected to the phase modulator l, as shown. The oscillations from O are phase modulated in l by the signal potentials from M and the phase modulated carrier is fed from I to the amplitude modulator AM, which may be supplied with other modulating potentials from a source M. The carrier frequency oscillations, which have already been modulated in phase by the signal potentials from! the source M, will in the stage AM be modulated in amplitude in accordance with the signal oscillations from M. The carrier frequency oscillations modulated in phase and in amplitude may be multiplied in frequency and amplified and radiated.

In order that the current in the arms of the phase modulating bridge circuit l of the present invention be maintained at minimum, a class A amplifier may be interposed between the amplitude modulator'AM and the phase modulator l, as shown.

Having thus described my invention and the operation thereof, what I claim is:

1. A phase modulator circuit comprising, a pair of thermionic tubes having their cathodes connected together by way of an impedance and their anodes connected together by way, of a second impedance equal in value to the value of the first impedance, means for applying high frequency oscillations to the cathode of one of said tubes and to the anode of another of said tubes, means for varying the impedances of said tubes in phase, and a utilization circuit connected with the cathode of the other of said tubes and the anode of said first named tube.

2. The combination with a bridge circuit having four arms and two pairs of opposed points of, a source of modulating potentials connected in phase to a pair of opposed arms, a source of high frequency oscillations connected across one of said pairs of said, opposed points, and a work circuit connected across the other pair of said opposed points.

3. Signalling means comprising, in combination, a bridge circuit having pairs of opposed impedances which are normally substantially numerically equal, one pair of which impedances are variable, and pairs of opposed points, a source of high frequency oscillations connected across one of said pairs of opposed points, a class A amplifier having its input electrodes connected across another pair of said opposed points, and means for phase modulating said high frequency oscillations comprising a source of modulating potentials connected with said opposed pair of variable impedances.

4. A combination as claimed in claim 3 including, a frequency multiplier connected with the output electrodes of said class A amplifier.

5. In a phase modulator, a bridge circuit having four balanced pedance, the impedances in an opposed pair of said arms being Variable, a circuit for applying carrier oscillations to a pair of opposed junction points on said bridge, a circuit for varying said variable impedances in phase at signal frequency,

and a relaying circuit connected with another pair of opposed junction points on said bridge.

6. A phase modulator circuit comprising, a pair of thermionic tubes having their cathodes connected together by way of a capacity and their anodes connected together by way of a second capacity substantially equal in value to the value of the first capacity, a circuit for applying high frequency oscillations to the cathode of one of said tubes and to the anode of another of said tubes, an inductance connected between the control grid and cathode of each of said tubes, a source of low frequency oscillations coupled to said inductances, and a utilization circuit connected with the cathode of the other of said tubes and the anode of said first named tube.

'7. A phase modulator circuit comprising, a pair of thermionic tubes having their cathodes connected together by way of a capacity and their anodes connected together by way of a second capacity substantially equal in value to the value of the first capacity, a circuit for applying high frequency oscillations to the cathode of one of said tubes and to the anode of the other of said tubes, a circuit for applying low frequency oscillations to the control electrodes of each of said tubes, an auxiliary electrode interposed between the anode and control electrode in each tube, and a utilization circuit connected with the cathode of the other of said tubes and the anode of said first named tube.

8. A phase modulator circuit comprising, a pair of thermionic tubes of the screen grid type having their cathodes connected together by way of a capacity and their anodes connected together by way of a second capacity equal in value to the value of the first capacity, a circuit for applying high frequency oscillations in phase opposition to the anode of one of said tubes and to the cathode of the other of said tubes, a circuit for applying oscillations of a different frequency in phase to the screen grid electrodes of said tubes, means for maintaining the control grid electrodes of said tube at a potential different with respect to the cathodes of said tubes, and a utilization circuit connected between the cathode of the other ofbsaid tubes and the anode of said first named tu e.

9. A phase modulator circuit comprising, a pair of thermionic tubes having their cathodes conarms, each including an im- 'lations connected across one nected together by way of a capacity and their anodes connected together by way of a second capacity equal in value to the value of the first capacity, a circuit for applying oscillations to the anode of one of said tubes and to the cathode of the other of said tubes in phase opposition, a circuit for applying oscillations of a different frequency in phase to the anodes of each of said tubes, means for maintaining the control grid electrodes of said tubes at a potential different than the potential of the cathodes, and a utilization circuit connected with the cathode of said first named tube and the anode of said other tube.

10. The combination, with a bridge circuit having four arms and two pairs of opposed junction points between said arms, of a source of modulating potentials coupled in phase to a pair of opposed arms, a source of high frequency oscilof said pairs of opposed junction points, an amplifier connected across the other pair of said opposed junction points, a frequency multiplier connected with said amplifier, and radiating means connected with said frequency multiplier.

11. The combination, with a bridge circuit having four arms, each including an impedance and two pairs of opposed junction points, of a source of modulating potentials coupled in phase to a pair of opposed arms, a source of high frequency oscillations connected across one of said pairs of said opposed junction points, an amplifier connected across the other pair of said opposed junc tion points, an amplitude modulator including a source of modulating potentials connected with said amplifier, and a utilization circuit coupled to said amplitude modulator.

12. The combination, with a bridge circuit having four arms and two pairs of spaced points, of a source of modulating potentials connected in phase to a pair of opposed arms, a source of high frequency oscillations connected with one of said pairs of said spaced points, and a class A amplifier connected across the other pair of said spaced points of said bridge.

13. Signalling means comprising, in combination, a bridge circuit having pairs of opposed impedances, one pair of which impedances is variable, and pairs of spaced points, a circuit for applying constant frequency oscillations to one of said pairs of spaced points, an amplifier having its input electrodes connected across another pair of said spaced points, and a circuit for applying modulating potentials in phase to said opposed pair of variable impedances.

14. A combination as set forth in claim 13 including, a frequency multiplier connected with the output electrodes of said amplifier.

15. In a phase modulator, a bridge circuit having four arms, each including an impedance, the impedances in an opposed pair of said arms being variable, a circuit for applying carrier oscillations to a pair of opposed points on said bridge, and a circuit for varying said variable impedances in phase at signal frequency whereby carrier wave energy modulated in phase at signal frequency is produced at another pair of opposed points on said bridge.

16. The combination of a device for phase modulating carrier waves including a bridge circuit having a plurality of arms, opposed pairs of said arms being substantially balanced, a circuit for applying carrier frequency oscillations to a pair of opposed junction points on said bridge circuit, means for phase modulating said carrier frequency oscillations comprising a circuit for applying modulating potentials to a pair of said opposed arms, an amplifier connected across another pair of opposed junction points on said bridge circuit, and an amplitude modulator connected with said amplifier.

17. The combination of a phase modulator in cluding a bridge circuit having a plurality of pairs of arms, opposed pairs of said arms being substantially balanced, a circuit for applying carrier frequency oscillations to a pair of opposed points on said bridge circuit, means for phase modulating said carrier frequency oscillations comprising a circuit for applying modulating potentials to a pair of opposed arms, an amplifier of the type in which small grid current flows during operation connected across another pair of opposed points on said bridge circuit, and an amplitude modulator including a circuit energized by modulating potentials connected with said amplifier.

18. A phase modulator comprising, a pair of thermionic tubes having their cathodes connected together by way of a reactance and their anodes connected together by way of a second reactance, a circuit for applying high frequency oscillations to the cathode of one of said tubes and to the anode of another of said tubes, a reactance connected between the control grid and cathode of each of said tubes, a source of low frequency potentials coupled in phase to said last named reactances, and a utilization circuit connected with the cathode of the other of said tubes and the'anode of said first named tube.

19. A phase modulator comprising, a pair of thermionic tubes of the screen grid type having their cathodes connected together by way of a reactance and their anodes connected together by way of a second reactance, a circuit for applying high frequency oscillations in phase opposition to the anode of one of said tubes and to the cathode of the other of said tubes, a reactance connected between the screen grid and cathode of each of said tubes, a circuit for applying oscillations of a different frequency to said last named reactances, and a utilization circuit connected between the cathode of the other of said tubes and the anode of said first named tube.

20. A phase modulator circuit including, a pair of thermionic tubes having their cathodes connected together by way of reactance and their anodes connected together by Way of a second reactance substantially equal in value to the value of the first reactance, a circuit for applying oscillations to the anode of one of said tubes and to the cathode of the other of said tubes in phase opposition, a reactance connected between the anode of each of said tubes and the cathode of each of said tubes, a circuit for applying controlling potentials in phase to said last named reactances, and a utilization circuit connected with the cathode of said first named tube and the anode of said other tube.

HENRY SHORE. 

